Rainbow Electronics MAX5069 User Manual
Page 11
hysteresis that allows the user to set a voltage (V
OFF
) to
disable the controller (see Figure 3).
At the beginning of the startup sequence, UVLO/EN is
below the 1.23V threshold, and Q1 turns on connecting
R
HYST
to GND (Figure 4). Once the UVLO 1.23V thresh-
old is crossed, Q1 turns off, resulting in the series com-
bination of R6, R
HYST
, and R7, placing the MAX5069 in
normal operating condition.
Calculate the turn-on voltage (V
ON
) by using the fol-
lowing formula:
where V
ULR2
is the UVLO/EN’s 1.23V rising threshold.
Choose an R
HYST
value in the 20kΩ range.
The MAX5069 turns off when the MAX5069 UVLO/EN
falls below the 1.18V falling threshold. The turn-off volt-
age (V
OFF
) is then defined as:
where V
ULF2
is the 1.18V UVLO/EN falling threshold.
Bootstrap Undervoltage Lockout
(MAX5069A/B)
In addition to the externally programmable UVLO func-
tion offered by the MAX5069, the MAX5069A/B feature
an additional internal bootstrap UVLO for use in high-
voltage power supplies (see the Functional Diagram).
This allows the device to bootstrap itself during initial
power-up. The MAX5069A/B start when V
IN
exceeds
the bootstrap UVLO threshold of 23.6V.
During startup, the UVLO circuit keeps the PWM com-
parator, ILIM comparator, oscillator, and output drivers
shut down to reduce current consumption. Once V
IN
reaches 23.6V, the UVLO circuit turns on both the PWM
and ILIM comparators, as well as the oscillator, and
allows the output driver to switch. If V
IN
drops below
9.7V, the UVLO circuit shuts down the PWM compara-
tor, ILIM comparator, oscillator, and output drivers,
returning the MAX5069A/B to the startup mode.
R
R
V
V
R
OFF
ULF
HYST
7
6
1
2
/
=
−
−
R
V
V
R
ON
ULR
HYST
6
1
2
=
×
−
MAX5069
High-Frequency, Current-Mode PWM Controller
with Accurate Oscillator and Dual FET Drivers
______________________________________________________________________________________
11
MAX5069A/D
1.23V
1.18V
UVLO/EN
R7
R6
V
IN
Figure 2. Setting the MAX5069A/D Undervoltage Lockout
Threshold
V
HYST
= V
ON
- V
OFF
V
OFF
V
ON
Figure 3. MAX5069 Hysteresis
MAX5069B/C
1.23V
1.18V
UVLO/EN
HYST
R
HYST
R6
R7
V
IN
Q1
Figure 4. Setting the MAX5069B/C Turn-On/Turn-Off Voltages